1. Field of the Invention
The present invention relates in general to the field of microwave devices and artificial dielectrics. More particularly, the present invention relates to the design and fabrication of highly transmissive and conductive frequency selective microwave devices using a resonant bulk material.
2. Background of the Invention
In applications ranging from commercial xe2x80x9cantenna farmsxe2x80x9d to military airborne active radars, the coupling of radiating structures to electromagnetic frequencies outside the desired frequency band or bands of operation remains a critical design limitation. In commercial communications applications, for example, high gain dish antennas must be encased in absorbing shrouds or radomes to prevent cross-coupling and interference with neighboring antennas broadcasting or receiving signals at different frequencies. In tactical aircraft, radars having a flat-plate array geometry for achieving high gain characteristics also makes such radars good scatterers of radiation at all other frequencies, particularly at frequencies above the active band of the radars.
Various solutions have been used to minimize the effects of cross-coupling in microwave devices, including Frequency Selective Surfaces (FSS""s), high performance tuning of the array, absorber loading and high squint angles, but all add complexity and cost to the system. FSS technology, for example, is notoriously difficult to adapt to complex geometries. such as aircraft radomes, and is also characterized by undesired out-of-band grating lobes. Other alternatives, such as Photonic Band Gap (PBG) materials, require complex and costly manufacturing techniques and very strict design and manufacturing tolerances.
As such, materials having a xe2x80x9ctuned electromagnetic window,xe2x80x9d obtained in bulk through chemical means and standard composite manufacturing processes, would present a cost effective solution to this problem. Frequency selective materials of two complementary types, judiciously applied to the above-described engineering problems would allow antennas and other microwave devices operating in different frequencies to share xe2x80x9creal estatexe2x80x9d with a minimal mutual coupling. Advantageously, a first type of such materials would remain opaque over most of a broad frequency band but transparent over a narrow frequency band window within the broad band, and a second type would remain transparent over most of a broad frequency band but highly reflective over a narrow frequency range within the broad band.
Therefore, a principal object of the present invention is to provide a variety of novel microwave devices that utilize bulk materials, such as narrow band metals (NARMET""s), having a xe2x80x9ctuned electromagnetic windowxe2x80x9d dependent on the electromagnetic characteristics of the material. A NARMET is herein defined as a condensed material, liquid or solid, whose permittivity exhibits a single sharp Lorentz resonance, i.e., a high Q, at a particular frequency or set of frequencies in the electromagnetic spectrum. It will be apparent to those skilled in the art that various microwave devices may be designed, tuned and manufactured for use in the diverse environments that exploit the resonance characteristics of NARMET materials.
More specifically, another object of the present invention is to provide a bulk material that is highly transmissive over a narrow band of desired frequencies. Such a material acts as an electromagnetic absorber over most of a broad frequency band while remaining electromagnetically transparent over a narrow band of desired frequencies with the broad band.
Still another object of the present invention is to provide a bulk material that is highly conductive over a narrow band of desired frequencies and thus behaves as a metal only with the desired frequency band. Such material is thus conductive and reflective over the narrow band of desired frequencies while remaining electromagnetically transparent at all other frequencies.
Hence, a class of microwave devices is disclosed that substantially overcomes the aforedescribed limitations and inadequacies of conventional microwave devices. A microwave device operating at a selected frequency band or group of selected frequency bands of interest is provided, for example, that is constructed and arranged using a resonant bulk material. The resonant bulk material includes a narrowband metal material having one or more conductivity bandwidths defined by one or more Lorentz resonance frequencies within the selected frequency band or group of selected frequency bands of interest. Accordingly, the narrowband metal material exhibits conductive properties at a frequency or frequencies within the one or more conductivity bandwidths and non-conductive properties at all other frequencies.
In another preferred embodiment, the resonant bulk material includes a dielectric matrix of lossy particles coated with the narrowband metal material, preferably liquid metal-ammonia. The coated particles are used to form highly transmissive microwave devices that absorb radiation at frequencies outside the one or more conductivity bandwidths of the narrowband metal material.
Alternatively, the resonant bulk material includes narrowband metal material is used to construct a variety of substructures that are used as part of wave guiding or wave scattering devices. Preferably, when the substructures are comparable in size, i.e., on the order of one-half the wavelength of the desired frequency or frequencies, the resonance of the device is enhanced.
Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.